A channel quality indicator (CQI) for indicating channel quality information to be described later in the present invention will hereinafter be described.
In order to implement an effective communication system, a receiver needs to inform a transmitter of feedback channel information. Generally, the receiver transmits downlink channel information through an uplink, and transmits uplink channel information through a downlink. This above-mentioned channel information is called a channel quality indicator (CQI).
The above-mentioned CQI can be generated in various ways. For example, channel state information is quantized without any change, so that the CQI can be transmitted using the quantized channel state information. A Signal to Interference and Noise Ratio (SINR) is calculated, and the CQI is transmitted according to the calculated SINR. And, the CQI may inform actual application status information of a channel in the same manner as in a Modulation Coding Scheme (MCS).
There are many cases for generating the CQI on the basis of the MCS in the above-mentioned CQI generation methods, so that their detailed description will hereinafter be described.
For example, the CQI can be generated for an HSDPA transmission scheme based on the 3rd Generation Partnership Project (3GPP). In this way, if the CQI is generated on the basis of the MCS, the MCS includes a modulation scheme, a coding scheme, and an associated coding rate, etc. Therefore, if the modulation scheme and the coding scheme are changed, the CQI must also be changed, so that a minimum number of CQI required for a codeword unit is at least 1.
If the MIMO scheme is applied to a system, the number of required CQIs is changed. In other words, the MIMO system generates multiple channels (i.e., a multi-channel) using multiple antennas (i.e., a multi-antenna), so that a plurality of codewords can be used for the MIMO system. As a result, the MIMO system must also use a plurality of CQIs. In this way, if many CQIs are used for the MIMO system, an amount of control information required for the CQIs proportionally increases.
FIG. 1 is a conceptual diagram illustrating a method for generating/transmitting the CQI.
Referring to FIG. 1, the user equipment (UE) 100 measures a downlink channel quality, selects a CQI value on the basis of the measured downlink channel quality, and reports the selected CQI value over an uplink control channel to the Node-B 200. The Node-B 200 performs downlink scheduling (e.g., UE selection, resource allocation, etc.) according to the reported CQI value.
In this case, the CQI value may be a Signal to Interference and Noise Ratio (SINR), a Carrier to Interference and Noise Ratio (CINR), a Bit Error Rate (BER), a Frame Error Rate (FER), or associated calculation value configured in the form of transmittable data. In the case of the MIMO system, Rank Information (RI) or Precoding Matrix Information (PMI) may be added to channel state information.
In the meantime, a mobile communication system employs a link adaptation to maximally use a channel capacity, and adjusts a Modulation and Coding Set (MCS) and a Transmission Power (TP) according to a given channel. In order to perform the above-mentioned link adaptation at the Node-B, the user equipment (UE) must feed back channel quality information to the Node-B.
If a frequency band used by the system has a bandwidth wider than a coherent bandwidth, a channel status is abruptly changed within an entire bandwidth.
Specifically, a multi-carrier system such as an Orthogonal Frequency Division Multiplexing (OFDM) system has a plurality of sub-carriers within a given bandwidth. The multi-carrier system transmits a modulated symbol via each sub-carrier, so that its optimum transmission is that the each subcarrier channel is considered when transmitting data.
Therefore, an amount of feedback channel information abruptly increases in the multi-carrier system including several sub-carriers, so that the demand of developing an improved method for reducing overhead of control signals is rapidly increased.